Cryptography is a word derived from the Greek that is approximately translated as hidden writing. There are various forms and styles of cryptography; however, since ancient times the aim of cryptography has been the same: secure the transmission of information from an emitter to a receiver. A classical form of cryptography involves the sharing of a code between the emitter and the receiver. A different algorithmic approach allows the use of a public key and is known as public key cryptography. This approach uses a public key and a secured key, both of which are mathematically connected. Quantum cryptography was first introduced by Weisner in 1983 and then by C. H. Bennett and G. Brassard. A refined protocol was introduced by Bennett et al. The Bennett and Brassard approach relies on the straightforward polarization property of single photons. An alternative method, based on the quantum entanglement of pairs of photons, was introduced by A. K. Ekert.
Cryptography is a word derived from the Greek that is approximately translated as hidden writing. There are various forms and styles of cryptography; however, since ancient times the aim of cryptography has been the same: secure the transmission of information from an emitter to a receiver. A classical form of cryptography involves the sharing of a code between the emitter and the receiver. A different algorithmic approach allows the use of a public key and is known as public key cryptography. This approach uses a public key and a secured key, both of which are mathematically connected. Quantum cryptography was first introduced by Weisner in 1983 and then by C. H. Bennett and G. Brassard. A refined protocol was introduced by Bennett et al. The Bennett and Brassard approach relies on the straightforward polarization property of single photons. An alternative method, based on the quantum entanglement of pairs of photons, was introduced by A. K. Ekert.
Quantum cryptography lies at the intersection of information theory and quantum mechanics. Cryptosystems come in two major classes depending on whether the key is shared in public or in secret. Public key cryptosystems are special in that they allow two people, who have not yet met, to communicate securely by looking-up encryption methods in a public directory. In quantum cryptography, the sender and receiver have two types of communication channels: a classical public channel and a quantum channel. The best application of cryptography is to allow a secure communication. In the near future, quantum cryptography may be used in top-secret applications in the military, for example, as the basis of communication links. The first protocol for quantum cryptography was proposed in 1984 by C. H. Bennett and G. Brassard. Some of the basic quantum cryptographic systems are: four-states protocol, two-states protocol, 4+2 protocol and multi-photon and multi-stage protocol.
Secure data exchange over the network is ensured by the requirements of confidentiality, integrity and availability of information. To meet these requirements, sent data is encrypted on the sending side, and the received information is decrypted on the receiving side. Transmitted message is encrypted according to rules determined by the algorithm and key. Crypto security of encrypted information depends on the key length. Among the encryption algorithms, there are algorithms with a symmetric (private) key and with an asymmetric (public) key. Despite all the advantages of a private key, asymmetric keys are used to deliver it from one user to another (key distribution). However, high-performance quantum computers are capable to decrypt intercepted data. Therefore, modern cryptographic systems use quantum key distribution method. The idea of using quantum bits was proposed in 1970 by S. Wiesner. In 1984, Ch. Bennett and G. Brassard proposed the BB84 protocol. The use of «entangled » quanta for systems with quantum key distribution was offerred in 1991 by A. Eckert. All modern quantum cryptography systems are based on these above-mentioned protocols.
Quantum Cryptography is based on the use of subatomic particles (photons) and their intrinsic quantum properties (photon polarization) to develop an unbreakable system. It is not possible to measure the quantum state of any system without affecting the system. This property provides secure transmission of key between sender and receiver. Quantum Cryptography is a way to combine the relative ease and convenience of key exchange in public key cryptography with the ultimate security of a one-time pad. Quantum techniques for key distribution, the classically impossible task of distributing secret information over an insecure channel whose transmissions are subject to inspection by an eavesdropper, between parties who share no secret key initially. Quantum Cryptography uses the principles of Quantum Mechanics to implement a cryptographic system. The key problem which is solved by using quantum techniques is that of eavesdropping detection. The bits are represented as qubits, physically modeled by photons, and communicated over a quantum channel. The polarization states of photons represent 0's and 1's. Keyword: Asymmetric key, Cryptography, Protocols, Quantum, symmetric key
The word cryptography originates from two Greek words: kryptos (secret) and graphein (writing). Thus cryptography is the art of communicating a message in secret manner or in other words, it is the art of rendering a message unintelligible to any unauthorized party. Its history is very old and exciting. From the beginning of human civilization there was a need to hide information from unauthorized people. This need gave birth to cryptography. Initial cryptographic protocols were very simple. With time they have become much more sophisticated and secure. However, all the classical cryptographic protocols developed so far are secure under some assumptions. In contrast, the protocols of quantum cryptography are unconditionally secure. This particular aspect of quantum cryptography makes it very interesting. This chapter aims to provide a flavor of the intrinsic beauty of various aspects of secure quantum communication. Before we start a journey into the fascinating world of secure quantum communication, it would be apt to become familiar with the jargon used in this field and also with some interesting protocols of classical cryptography. Let us first do that.
Cryptographic protocols can be classified by the type of security against eavesdropping which they provide. There exist mathematically secure schemes whose security relies on mathematical proofs (like the Vernam cipher discussed below) or conjectures (like public key RSA encryption) about the complexity of deciphering the message without possessing the correct key. The majority of current secure public Internet connections rely on such schemes. Alternatively, a cryptographic setup may provide a physically secure method for communicating. In these setups the security is provided by the physical laws governing the communication protocol. Here we first discuss a provably secure classical communication protocol and then quantum methods for distributing the necessary keys. The BB84 protocol relies on the impossibility of perfectly distinguishing non-orthogonal quantum states from one copy of the quantum system. The scheme was invented in 1984 and is the first of its kind. In contrast, the Ekert91 protocol makes use of Bell correlations between entangled pairs of photons. These correlations are destroyed when Eve attempts to make a measurement on one of the particles but also when imperfections, such as decoherence processes, affect the scheme. As long as no such “elements of reality” are introduced, and Bell correlations violating local realism can be generated by Alice and Bob, no eavesdropper can be present. One-time pads and the Vernam cipher The Vernam cipher is a cryptographic protocol which allows the encryption and decryption protocol to be publicly known. The security of the protocol relies entirely on the key which is private and not publicly known. Alice and Bob share identical n -bit secret key strings (one-time pad).
Cryptography is a method of secure communication between two or more parties. The crucial step is exchanging a key in a secure manner. There are, however, two problems with conventional cryptography. First the sender and the receiver should exchange the key through highly reliable and secure channels. The second problem is that a clever eavesdropper can, by a careful analysis of the sent information, reconstruct the key. In this chapter, schemes to overcome these problems are presented. First a scheme for exchanging a key over public channels, the so-called RSA algorithm, is discussed. Then the protocols for the quantum key distribution (QKD), the Bennett–Brassard-84 (BB-84) and Bennett-92(B-92) protocols, are then presented. The QKD protocols are exclusively derived using Bohr’s principle of complementarity. An application of these ideas to the design of secure quantum money is discussed.